Ana the Ist
Aggressively serene!
I already answered the other part of your question. Atheism claims it's reasonable to lack belief in God, but in reality atheists either believe God does not exist or believe God is possible. There's no lacking belief in the concept of God, you either accept it as false or you accept it as true or at least possible.
I can see how it might look this way to a theist...but you really need to try to see it from the atheists' point of view. Imagine this...
Someone is telling you that something exists...but they can't really tell you what that thing is. Instead of telling you what it is, they give you these vague and confusing descriptions of it. Some are useful (god is the creator of the universe) some aren't clearly defined concepts (god is timeless/omnipresent/omnibenevolent) some are flat out nonsensical and contradictory (god is love/truth/knowledge). After this person is done describing this incomparable entity to you...they ask you if you believe it exists.
At that point, it's entirely appropriate to tell them that you lack a belief in their god. After all, you can't really make a decision on the existence of something that you don't even fully understand (something that the theist will often admit of himself). If the theist were giving you a more complete picture of what his god actually is...perhaps you could give him an answer regarding its existence. Until such a time though, it's more accurate to say that you "lack a belief in god".
Hope that helps.
Upvote
0